Health checks in Compose in Docker - Practice Problems & Coding Challenges

Challenge - 5 Problems

💻 Command Output

intermediate

Health check status output in Docker Compose

Given this Docker Compose service with a health check, what will be the output of docker-compose ps after the container starts and the health check passes?

Docker

services:
  web:
    image: nginx
    healthcheck:
      test: ["CMD", "curl", "-f", "http://localhost"]
      interval: 10s
      timeout: 5s
      retries: 3

A web docker-entrypoint.sh nginx - Up (unhealthy)

B web docker-entrypoint.sh nginx - Up (healthy)

C web docker-entrypoint.sh nginx - Up (starting)

D web docker-entrypoint.sh nginx - Exited (0)

💡 Hint

Think about what the health check does and how Docker reports container health.

The health check runs curl to check if the web server responds. If it succeeds, Docker marks the container as healthy, so the status shows 'Up (healthy)'.

🧠 Conceptual

intermediate

Purpose of health checks in Docker Compose

What is the main purpose of defining a health check in a Docker Compose service?

A To expose the container's ports to the host

B To automatically restart the container if it crashes

C To limit the container's CPU and memory usage

D To monitor if the container's application is working correctly

💡 Hint

Think about what health means for a running service.

Health checks let Docker know if the app inside the container is running properly, beyond just the container being up.

❓ Configuration

advanced

Correct health check syntax in Docker Compose

Which of these Docker Compose health check configurations is syntactically correct and will run a command every 30 seconds with a 10-second timeout?

A

healthcheck:
  test: ["CMD", "curl", "-f", "http://localhost"]
  interval: 30s
  timeout: 10s

B

healthcheck:
  test: curl -f http://localhost
  interval: 30s
  timeout: 10s

C

healthcheck:
  test: curl -f http://localhost
  interval: 30
  timeout: 10

D

healthcheck:
  test: ["curl", "-f", "http://localhost"]
  interval: 30s
  timeout: 10s

💡 Hint

The test command must be an array starting with CMD for Compose health checks.

Option A uses the correct array syntax with CMD and valid interval and timeout units. Others have syntax errors or missing CMD.

❓ Troubleshoot

advanced

Diagnosing failing health checks in Compose

You have a health check in Compose that always reports 'unhealthy'. Which command helps you see the detailed health check logs for the container?

A docker-compose logs --follow

B docker-compose ps

C docker inspect --format='{{json .State.Health}}' <container_id>

D docker stats <container_id>

💡 Hint

You want to see the health check results stored in container state.

The docker inspect command with .State.Health shows health check logs and status details.

🔀 Workflow

expert

Using health checks to control service startup order

In Docker Compose, how can you ensure that a service app starts only after the db service is healthy?

A Use <code>depends_on:</code> with <code>condition: service_healthy</code> for <code>app</code> on <code>db</code>

B Use <code>depends_on:</code> with <code>condition: service_started</code> for <code>app</code> on <code>db</code>

C Start <code>db</code> manually before <code>app</code> using separate commands

D Set <code>restart: always</code> on <code>app</code> service

💡 Hint

Compose supports waiting for health status in depends_on since version 3.4.

Using depends_on with condition: service_healthy makes Compose wait for the db service to be healthy before starting app.